In the literature there are lot of types of pumps and pump impellers for pumping fluids such as sewage water. However, all of these pumps have certain disadvantages relating to clogging and low efficiency.
Sewage water contains a lot of different types of pollutants, the amount and structure of which depend on the season and type of area from which the water emanates. In cities, plastic material, hygiene articles, textile etc are commonly found in the sewage water. Industrial areas produce sewage water with wearing particles. Experience shows that the worst problems are rags and the like which stick to the leading edges of the vanes and become wound around the impeller hub. Such incidents cause frequent service intervals and a reduced efficiency.
In agriculture and pulp industries, different kinds of special pumps are used to manage straw, grass, leaves and other types of organic material. For this purpose the leading edges of the vanes are swept backwards in order to cause the pollutants to be fed outwards to the periphery instead of getting stuck to the edges. Different types of disintegration means are often used for cutting the material and making the flow more easy. Examples are shown in Swedish patents SE-435 952, SE-375 831 and U.S. Pat. No. 4,347,035.
As pollutants in sewage water are of other types and thus, more difficult to master, and as the operation times for sewage water pumps are normally much longer, the above mentioned special pumps do not fulfill reliability or efficiency requirements when pumping sewage water.
A sewage water pump quite often operates up to 12 hours a day which means that the energy consumption depends a lot on the total efficiency of the pump.
Tests have proven that it is possible to improve efficiency by up to 50% for a sewage pump according to the invention as compared with known sewage pumps. As the life cycle cost for an electrically driven pump normally is totally dominated by energy costs (c:a 80%), thus it is evident that such a dramatic increase will be extremely important.
The designs of pump impellers are described very generally in the literature, especially in regard to the sweep of the leading edges. An unambiguous definition of sweep does not exist.
Tests have shown that the design of the sweep angle distribution on the leading edges is very important in order to obtain the necessary self cleaning ability of the pump impeller. The nature of the pollutants also calls for different sweep angles in order to provide a good function.
The literature does not give any information about what is needed in order to obtain a gliding transport of pollutants outwards in a radial direction along the leading edges of the vanes. Generally what is mentioned is that the edges of the vanes shall be obtuse-angled, swept backwards, etc. See Swedish patent SE-435 952.
When smaller pollutants such as grass and other organic material are pumped, relatively small angles may be sufficient in order to obtain radial transport and also disintegrate the pollutants in the slot between pump impeller and the surrounding housing. In practice, disintegration is obtained by the particles being cut through contact with the impeller and the housing when the former rotates with a periphery velocity of 10 to 25 m/s. This cutting process is improved by the surfaces being provided with cutting devices, slots or the like. Compare Swedish patent SE-435 952. Such pumps are used for transport of pulp, manure etc.
When designing a pump impeller having vane leading edges swept backwards in order to obtain self cleaning, a conflict arises between the distribution of the sweep angle, performance and other design parameters. In general, it is true that an increased sweep angle means less risk for clogging, but at the same time efficiency decreases.